Digitizer tablets are well known in the art. In one popular type, the pointing device comprises a coil in a cursor or stylus configuration which is positioned by a user over a tablet surface in which is embedded a wire grid extending in two coordinate directions. In one mode, the pointing device coil is energized to electromagnetically induce signals in the grid wires. In another mode, the grid wires are energized to electromagnetically induce signals in the pointing device coil. In both modes, the grid wires are sequentially addressed, either one at a time or in groups, to provide an output analog voltage in timed relation to the grid addressing. The output voltage with increasing time reaches a maximum, passes through zero, and then reaches a minimum in the vicinity of the pointing device coil. When addressing begins, a counter is activated which keeps track of the particular grid line addressed. A stop signal for the counter is generated when the zero crossover is determined. The count value determines the location of the pointing device with respect to the two or three closest grid wires. Examples of patents describing in more detail this type of digitizer are Kamm et al U.S. Pat. No. 3,904,822; Ioanau U.S. Pat. No. 3,873,770; and Zimmer U.S. Pat. No. 4,368,351, whose contents are hereby incorporated by reference.
In a typical tablet, the stylus or cursor position is expressed as an X, Y coordinate pair, with the X and Y axes having their normal orientation from an origin at the lower, left-hand corner of the tablet. The Z axis in this coordinate system is upward from the tablet surface. For most tablets, the Z axis represents only proximity, which is the maximum distance above the tablet's active area that the cursor or stylus can be held and still report a valid position. Thus, the Z coordinate is typically represented only as one of two binary values. Another way of utilizing the Z axis is as a binary switch. In this mode, a switch (sometimes called a button) is incorporated inside the barrel of the stylus and is connected to the stylus tip. The switch is normally open and is activated by the user pressing the stylus tip against the tablet. The switch action can be used in the same manner as an outside button on the stylus or as the buttons on a cursor.
But there also exist tablets in which Z coordinate data is employed as a kind of analog switch. This is implemented by incorporating in the stylus in place of the switch a linear transducer connected to the stylus tip. In this mode, the Z axis output is now a function of the stylus tip pressure, that is, the amount of force that the user employs in pressing the stylus against the tablet surface. The advantage is that now a third variable becomes available for use by an application program to enhance the user interface. A number of application programs use this Z coordinate data to perform certain functions. For example, one line drawing or paint program uses the stylus pressure to determine paint brush or line width, with the heavier the pressure, the wider the line. Another program uses stylus pressure for zooming icons pointed to by the stylus.
As mentioned, implementation of Z coordinate data is accomplished by incorporating a linear transducer in the stylus barrel. Known linear transducers used are mechanical strain gauges, or gel pellets whose resistance changes as the gel is compressed when the user presses the stylus tip against a surface. It is desirable for the stylus to generate Z data over a reasonably wide range of stylus pressure. A typical range is from 0-6 oz of pressure. Assuming a resolution of, say, 0.02 oz, this would allow a total of 300 distinguishable Z values to be detected and used by the application program. But with a linear transducer, this would require a user to be able to control the stylus so as to apply fifty different pressures not only in the 0-1 oz range at the low end, but also in the 5-6 oz range at the high end. This imposes undue demands on user dexterity and control.